A conventional quadrupole mass spectrometer consists of four parallel electrodes to which RF and DC voltages are applied. The electrode profiles may be either hyperbolic (which produces ideal quadrupolar in between the rods) or round (which with the correct spacing produces a close approximation to ideal fields). Quadrupole mass spectrometers are widely used in commercial mass spectrometer systems for trace chemical analysis.
The quadrupole operate as mass filter when a beam of ions passes along the axis by allowing only a selected mass range (or more correctly, m/e where m is the mass and e is the charge on the ion) to be transmitted. Only ions which are stable can be transmitted, the condition for stability being defined by the non-dimensional parameters a and q:q=4eV/(r02ω2m)a=2eU/(r02ω2m)where U is the DC voltage, V is the RF voltage, r0 is the radius of the inscribed circle between the rods, ω is the angular frequency of the RF voltage (radians/s), m is the mass of the ion and e is the charge. Ions which have a and q values outside the limits of stability increase their amplitude of oscillation and are lost to the rods.
Typically a mass spectrum is obtained by sweeping the RF and DC voltages (in a fixed ratio) through a range of values so that ions of increasing mass pass through the same a−q value of stability and are sequentially transmitted.
It will be noted that the same effect can be achieved by sweeping the frequency, although there is a square relationship between mass and frequency.